The invention relates to a coverter for a photographic, slide, or cinematographic negative-to-television conversion, comprising a pick-up device for picking up a photographic, slide or cinematographic negative with picture contents and for supplying a picture pick-up signal, and a picture signal processing circuit for processing the picture pick-up signal to provide a picture display signal suitable for display on a television display screen, in which signal processing operation the picture signal is inverted and fixed at a black level and a white level.
A converter of this type provides the possibility of showing photographic negatives in a positive form on the television display screen. For fixing the picture signal at the correct black and white levels, these levels must be set or adjusted. Starting from a picture pick-up signal YP, a picture display signal YD must be formed in accordance with a relation YD=C-K.times.YP, in which C and K are constants. Assuming that a standardized black level is equal to 0 and a standardized white level is equal to 1, it follows that for a white level (YD=1) to be displayed, the picked-up black level (YP=0) is adjusted by means of the constant C (YD=C=1). Subsequently the black level (YD=0) to be displayed is adjusted by adjusting the constant K (YD=1-K=0) in the relation YD=1-K.times.YP with the picked-up white level (YP=1).
The setting or adjustment of the two constants C and K holds for monochrome television. In the case of color television with the three primary colors red (R), green (G) and blue (B) the adjustment must be effected three times, because the relations RD=Cr-Kr.times.RP, GD=Cg-Kg.times.GP and BD=Cb-Kb.times.BP must be satisfied. Furthermore it holds for color television that the black level Y=0 is present for the luminance signal Y=0.3R+0.59G+0.11B with R=G=B=0, and the white level Y=1 is present with R=G=B=1.
Setting or adjusting the two constants in monochrome television is required for obtaining the correct black and white in the displayed picture and the correct contrast range therein. Moreover, it is essential for color television to correctly set or adjust all six constants for obtaining the correct colors upon display. In practice it is found that, starting from the displayed color picture, it is substantially impossible to find the correct six constant values by way of experiment due to the mutual influences of displayed color and luminance. In addition, the photographic films made by the various film manufacturers require different, adapted values for the constants, while each film type has its own spread.